Joe Arpaio is also the subject of a civil lawsuit brought by the justice department, in a broader case accusing him of civil rights abuses. Photograph: Ross D Franklin/AP

The self-proclaimed "toughest sheriff in America", Joe Arpaio, will be called to account in a courtroom this week over long-standing accusations that he is waging an unlawful campaign of discrimination and harassment against Latinos.

Arpaio, the sheriff of Maricopa County, Arizona, will be questioned before a federal judge in Phoenix in a class action lawsuit over allegations that his so-called "crime suppression" sweeps racially profile Latinos and those who look like them rather than target any evidence of criminal activity.

The sheriff, who has endorsed Mitt Romney for president and was in the audience at the televised Republican debate in Arizona, is already the subject of a civil lawsuit brought by the justice department, in a similar but broader case accusing him of widespread civil rights abuses.

The plaintiffs, a small group claiming discrimination and harassment by Arpaio and his deputies, say they have "voluminous evidence" alleging that the sheriff's office in Maricopa County is engaged in a systematic practice of racial profiling. They allege that some sweeps or "saturation patrols" were prompted by letters from the public warning of workers "speaking only Spanish" and about "dark skinned" individuals.

One letter to Arpaio's office warning of McDonalds staff speaking only in Spanish to one another has been scribbled over, in what lawyers for the plaintiff say is Arpaio's handwriting, instructing "For our operations". Another letter, which refers to "illegals" "crawling around here" and reads: "If you have dark skin, then you have dark skin! Unfortunately, that is the look of the Mexican illegal who are here ILLEGALLY" was circulated by Arpaio to staff, lawyers said.

Andrew Byrnes, an attorney with Covington and Burling LLP, representing the plaintiffs, along with the Mexican American Legal Defense and Educational Fund, said Arpaio had made inflammatory public comments equated illegal immigrants with Latinos and that he was using the saturation patrols as a "drag net" for illegal immigrants.

Byrnes said: "The sheriff has notated 'to our operations' to staff involved in these sweeps, notating that actions should be taken on them. In many cases, law enforcement actions in these sweeps are taking place shortly after the sheriff has passed these (letters) to his team."

He said Arpaio "has endorsed explicit calls for racial profiling sent to him" by circulating such letters to his staff. Data provided by the sheriff's office shows that Hispanics are stopped at a significantly greater rate than others, he said.

During the sweeps or "saturation patrols" in Arpaio's Maricopa County, his deputies target an area of the city – in some cases, heavily Latino areas – to seek out traffic violators and other offenders.

Arpaio denies racial profiling allegations, saying that people targeted in the sweeps were approached because they were suspected of crimes.

Dr Ralph Taylor, a criminal justice expert at Temple University and expert witness for ACLU, has analysed data from Maricopa County sheriff's office. He has found "statistically significant" evidence that officers were significantly more likely to stop Latino individuals on "saturation patrol" (SAT) days in comparison to normal days. He also found that officers on SAT patrols were 46% to 54% more likely to stop Latinos than officers not involved in SAT patrols and that, if a Latino was stopped, it took Maricopa officers longer to complete the traffic stop.

The plaintiffs in the case are not seeking damages but are seeking an injunction to stop what they claim is harassment and discrimination against them.

One plaintiff, Manuel de Jesus Ortega Melendres, a legal visitor to the United States with a valid visa, was arrested by Arpaio's deputies in 2007, when they pulled over a car he was travelling in. He provided identification but was jailed for four hours before being released. The driver, who was white, was not given a citation or taken into custody. Another plaintiff, Manuel Nieto, was thrown against his car, and he and his sister, Velia Meraz, were held at gunpoint in front of their family business during a sweep in which they were stopped. They are both US citizens. Another woman, who is not a plaintiff but a declarant in the case, is Diane Solis. Solis, a US citizen, was travelling with her young son and three other children back from trip to the Grand Canyon in March 2009 when she and children were detained.

The case will begin on Thursday and is expected to last until 2 August.